This invention relates to aldol condensation and in particular to the condensation of straight chain aldehydes such as propanal, butanal, or hexanal.
The aldol condensation of propanal or butanal is an important step in the production of oxo alcohols such as 2-ethyl hexanol. For example, as described in GB 1462328, butanal is typically condensed using an aqueous caustic catalyst at a temperature of the order of 80 to 140xc2x0 C. to give 2-ethyl hexenal which is then hydrogenated to 2-ethyl hexanol.
We have found that the condensation can be effected in the gaseous phase using a solid base catalyst thereby avoiding the need for aqueous caustic solutions with their consequent handling and effluent disposal problems.
U.S. Pat. No. 5,144,089, U.S. Pat. No. 5,254,743 and U.S. Pat. No. 5,258,558 describe the hydrogenation of butanal in the liquid phase using fixed bed catalysts based on magnesia-containing compositions. However it is desirable to devise a gas phase process since such a process would be easier to back integrate into the gas phase OXO hydroformylation process that is used to produce aldehydes such as butanal and so energy savings could be achieved by eliminating an aldehyde vaporisation process. It would appear from these U.S. patents that the magnesia based catalysts require activation by heating at high temperatures in an inert atmosphere before use. Such a pre-treatment step is undesirable. Also it would be desirable to provide a process employing catalysts that may be more selective than the aforesaid magnesia-containing catalysts.
The aforementioned U.S. patents also mention the use of fixed bed catalysis using sodium on alumina or potassium on graphite. It is believed that this is a reference to the type of catalysts described by Pines et al in xe2x80x9cBase-catalysed reactions of hydrocarbons and related compoundsxe2x80x9d, Academic Press, 1977, pages 18-20 in which the alkali metal was in the elemental form and which were generally used at low temperatures for the liquid phase reactions. These catalysts are unsuitable for aldol condensations since they would be rapidly de-activated by the water produced in the condensation reaction. Also such catalysts are liable to lead to low product selectivity.
U.S. Pat. No. 5,453,412 and U.S. Pat. No. 5,498,587 describe the hydrogenation of butanal using certain copper catalysts containing sodium oxide at relatively low temperatures, below 160xc2x0 C. We have found that higher temperatures are necessary in order to effect aldol condensation with solid base catalysts. Thus under the conditions described in the Examples of those U.S. patents, essentially no aldol condensation took place.
Accordingly the present invention provides a process for the production of unsaturated aldehydes by the aldol condensation of straight chain aldehydes by contacting the aldehyde in the vapour phase with a particulate catalyst comprising at least one basic alkali metal compound supported on an inert substrate at a temperature above 175xc2x0 C.
Suitable catalysts are basic sodium, potassium, or cesium compounds such as oxides hydroxides or carbonates supported on a material such as carbon, silica, alumina, a clay, silicalite or a zeolite. Preferred catalysts are alkali metal compounds supported on silica, especially potassium or sodium supported on silica. The potassium and sodium catalysts appear to have high activity and are the most selective. The catalyst preferably contains 0.1 to 25%, preferably 0.4 to 18%, by weight of the alkali metal.
The support preferably is in the form of particles having maximum and minimum dimensions in the range 0.5 to 10 mm, preferably 1 to 4 mm, and having a BET surface area in the range 50 to 500 m2/g. The catalyst is preferably made by impregnating the support particles with an aqueous solution of an alkali metal compound that is basic or decomposes to a basic compound upon heating, for example an alkali metal hydroxide, acetate, oxalate, nitrate or carbonate, followed by drying and calcination if necessary to effect decomposition to a basic compound.
The reaction is effected at temperatures above 175xc2x0 C., particularly above 200xc2x0 C., and preferably below 450xc2x0 C., particularly in the range 200 to 350xc2x0 C. As the temperature increases the activity increases but the selectivity tends to decrease, often with the production of hydrogenated products.
The aldehyde is preferably a straight chain aldehyde containing 2 to 8 carbon atoms, preferably, propanal, butanal, or hexanal.
After a period of operation, the activity of the catalyst tends to decrease through the deposition of carbon as a result of side reactions. The catalyst may be periodically regenerated by burning off the carbon by heating in an oxygen-containing atmosphere, e.g. air or oxygen or air diluted with an inert gas such as nitrogen. The catalyst may be disposed as a fixed bed or a fluidised bed may be employed. In the latter case a portion of the catalyst may be continuously withdrawn and regenerated and returned to the reaction zone.
The main product of the condensation is an unsaturated aldehyde, e.g. 2-ethyl hex-2-enal in the case of condensation of butanal, or 2-butyl oct-2-enal in the case of hexanal. Often it is desired to hydrogenate the product to the corresponding alcohol, e.g. 2-ethyl hexanol, or 2-butyl octanol. This may be effected by passing the products, possibly after separation of the starting aldehyde that has not reacted, together with hydrogen, through a bed of a suitable hydrogenation catalyst, such as copper or a platinum group metal, on a suitable support. The temperature of the hydrogenation is effected will often be below that used for the aldol condensation. The reaction mixture from the aldol condensation may be cooled to the desired hydrogenation temperature by addition of a suitable quench gas, such as cool hydrogen.
In some cases it may be desired to produce a mixture of the alcohol resulting from hydrogenation of the aldol condensation and the alcohol resulting from hydrogenation of the feed aldehyde. For example it may be desired to produce a mixture of 2-ethyl hexanol and butanol. Where this is desired, separation of the condensation product from the feed aldehyde prior to hydrogenation will generally not be necessary. Indeed in some cases it may be desirable to provide a bypass so that part of the feed aldehyde may bypass the aldol condensation step. In this case, the bypass aldehyde may be cooled so that it may be used as part or all of the quench gas. Where such a bypass is employed, variation of the amount of bypassing aldehyde may be used to control the relative proportions of the products, e.g. to compensate for declining activity of the base catalyst.
As indicated above, the primary product from the condensation is the unsaturated aldehyde, e.g. 2-ethyl hex-2-enal or 2-butyl oct-2-enal. In some cases, the desired product is not the corresponding alcohol but is the corresponding saturated aldehyde, e.g. 2-ethyl hexanal or 2-butyl octanal. The unsaturated aldehyde may be hydrogenated to the saturated aldehyde using a hydrogenation catalyst, such as palladium, that effects hydrogenation of the carbonxe2x80x94carbon double bond but does not effect hydrogenation of the carbonyl group.
In some cases it may be possible to effect the condensation and hydrogenation in a single stage by formulating the base catalyst also to have the appropriate hydrogenation activity and co-feeding hydrogen with the aldehyde to the reaction zone. Such a base/hydrogenation catalyst may be a mixture of separate particles of base and hydrogenation catalyst, or may be particles of the support impregnated with both a base and a material having hydrogenation activity. However, it has been found that where the condensation and hydrogenation is effected in a single stage, e.g. by the use of a catalyst having both the condensation and hydrogenation activity, the condensation activity of the catalyst may decrease relatively rapidly. Therefore it is preferred to effect such condensation and hydrogenation in separate stages, e.g. by using a bed of the condensation catalyst, followed by a bed of the hydrogenation catalyst. In this case it is preferred that the condensation catalyst is free from components, such as copper, and Group VIII metals, giving hydrogenation activity.
The invention is illustrated by the following examples.